The present invention relates to aluminum or aluminum alloy surfaces which are treated with corrosion resistant ceramic type compounds so as to be useful as dielectrics and substrates for subsequently applied coatings. More particularly, the hydrophilic surfaces thusly produced are suitable for use as base supports for lithographic printing plates.
Heretofore, in the production of metal presensitized lithographic printing plates, it had been found beneficial to treat the surface of the metal substrate sheet, with a protective interlayer substance which imparts beneficial characteristics to the final lithographic printing plate thus produced. The prior art teaches that it is desirable to treat the metal sheet substrate surface receiving the light sensitive coating material, which when exposed to light and developed becomes the printing surface of the printing plate, with an undercoating substance that hydrophilizes the substrate and forms a strong bond with the metal sheet substrate and with the light sensitive coating material.
Many such undercoating treatments are known in the art for manufacturing longer running lithographic plates. U.S. Pat. Nos. 3,160,506, 3,136,636; 2,946,683; 2,922,715 and 2,714,066 disclose a variety of suitable materials for undercoating bonding substances onto plates and methods for applying them. Alkali silicate, polyvinyl phosphonic acid, silicic acid, alkali zirconium fluoride and hydrofluozirconic acid solutions presently are the most important commerical bonding substances. Those materials substantially improve the bonding of the light sensitive coating to the underlying metallic base which otherwise generally tends to have inadequate affinity for the coating.
The application of silicates both electrically and thermally, is well known to be a method of producing a ceramic-like layer on aluminum and its alloys which is non-porous and hydrophilic and is particularly useful for wipe-on plates and to a lesser degree, presensitized lithographic printing plates. The advantages most realized in the silicate process are the quick roll-up due to the glass-like nature of the surface and the ability to set for extended periods of time without loss of hydrophilicity before the photosensitive coating is applied. However, due to the alkaline nature of the sodium silicate used, it is not always possible to have a consistently good presensitized printing plate, even when well rinsed, and then coated with diazonium compounds.
Also, rinsing is critical especially in the case of thermal silication where copious amounts of water are needed. Electrosilication is more forgiving in that a mild acid rinse may be used. Finally, sodium silicate may not be made acidic since an insoluble silisic acid precipitate is formed.
Various borates, phosphates and the fluoro derivatives thereof are also known to be useful when thermally applied. U.S. Pat. No. 4,153,461 teaches that aqueous solutions of organic acids are useful in the production of substrates which form the base of lithographic printing plates. The most preferred such acid is polyvinyl phosphonic acid.
Polyvinyl phosphonic acid treatment offers the advantage of producing a surface that is acidic and therefore inherently compatible with diazonium compounds. Both thermal and electrical techniques provide better adhesion between the aluminum and applied light sensitive coating which translate into better press performance. The advantages of such compounds are that they provide chemical bonding to the aluminum and diazonium compounds in the coating, by covalent bonding in the former case and ionic bonding in the latter, and that they result in presensitized lithographic printing plates having excellent shelf lives. Some disadvantages of surfaces prepared with these compounds are: (1) the prepared surface can not set too long between the time it is manufactured and it is coated; (2) the inherent hydrophilicity is not as great as silicated plates; (3) the ability to roll up clean and remain clean, particularly after the press has run and then shut down, is not always realized.
It is an object of the present invention to provide a technique whereby the aforesaid advantages of both the acid and ceramic treatments are substantially attained and the undesirable features are substantially negated.